Combination of searches for WW , WZ , and ZZ resonances in pp collisions at √s = 8 TeV with the ATLAS detector

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Article

Reference

Combination of searches for WW , WZ , and ZZ resonances in pp collisions at √s = 8 TeV with the ATLAS detector

ATLAS Collaboration

ANCU, Lucian Stefan (Collab.), et al.

Abstract

The ATLAS experiment at the CERN Large Hadron Collider has performed searches for new, heavy bosons decaying to WW , WZ and ZZ final states in multiple decay channels using 20.3 fb−1 of pp collision data at s=8 TeV . In the current study, the results of these searches are combined to provide a more stringent test of models predicting heavy resonances with couplings to vector bosons. Direct searches for a charged diboson resonance decaying to WZ in the ℓνℓ′ℓ′ ( ℓ=μ,e ), ℓℓqq¯ , ℓνqq¯ and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WZ bosons are compared with predictions of an extended gauge model with a heavy W′ boson. In addition, direct searches for a neutral diboson resonance decaying to WW and ZZ in the ℓℓqq¯ , ℓνqq¯ , and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WW and ZZ bosons are compared with predictions for a heavy, spin-2 graviton in an extended Randall–Sundrum model where the Standard Model fields are allowed to propagate in the bulk [...]

ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Combination of searches for WW , WZ , and ZZ resonances in pp collisions at √s = 8 TeV with the ATLAS detector. Physics Letters. B , 2016, vol. 755, p. 285-305

DOI : 10.1016/j.physletb.2016.02.015

Available at:

http://archive-ouverte.unige.ch/unige:81239

Disclaimer: layout of this document may differ from the published version.

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Combination of searches for W W , W Z , and Z Z resonances in pp collisions at √

s = ^{8 TeV with} ^the ^ATLAS ^detector

.ATLASCollaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received16December2015

Receivedinrevisedform9February2016 Accepted9February2016

Availableonline11February2016 Editor:W.-D.Schlatter

TheATLASexperimentattheCERNLargeHadronColliderhasperformedsearchesfornew,heavybosons decayingto W W,W Z and Z Z ﬁnalstatesinmultipledecaychannelsusing20.3 fb⁻¹ ofpp collision dataat√

s=^{8 TeV.}În^the^current^study,^the^resultsôf^these^searchesâre^combined^to^provideâ^more stringenttest ofmodelspredictingheavy resonanceswithcouplingsto vectorbosons.Directsearches forachargeddibosonresonancedecayingtoW Z intheν(=μ,e),q^q,¯ νq^q¯ ând^fully^hadronic finalstatesare combinedandupperlimits ontherateofproductiontimesbranchingratiototheW Z bosonsarecomparedwithpredictionsofanextendedgaugemodelwithaheavyWboson.Inaddition, directsearchesforaneutraldiboson resonancedecayingtoW W and Z Z intheq^q,¯ νq^q,¯ ând^fully hadronicfinalstatesarecombinedandupperlimitsontherate ofproductiontimesbranchingratioto the W W and Z Z bosonsarecompared withpredictionsfor aheavy,spin-2 gravitoninan extended Randall–Sundrummodel wheretheStandardModel fieldsareallowedtopropagateinthe bulkofthe extradimension.

©²⁰¹⁶^CERN^for^the^benefitôf^theÂTLASCollaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Thenaturalnessargumentassociatedwiththesmallmassofthe recentlydiscoveredHiggs boson[1–4] suggeststhat theStandard Model(SM)isconceivablytobeextendedbyatheorythatincludes additionalparticles and interactions atthe TeV scale. Many such extensionsoftheSM,suchasextendedgaugemodels[5–7],mod- elsofwarpedextradimensions[8–10],technicolour[11–14], and more generic composite Higgs models [15,16], predict the exis- tenceofmassiveresonancesdecayingtopairsofW andZ bosons.

Intheextendedgaugemodel(EGM)[5]anew,chargedvector boson(W)couplestotheSMparticles.Thecouplingbetweenthe WandtheSMfermionsisthesameasthecouplingbetweenthe W bosonandtheSMfermions.TheWW Z couplinghasthesame structureasthe W W Z couplingintheSM,butisscaledbyafac- torc×(mW/m_W)²,where c isa scalingconstant,mW isthe W boson mass,andm_W is the W boson mass.The scaling of the coupling allows the width of the W boson to increase approx- imately linearly withmW at mW ^mW and to remain narrow forc∼^1.^For^c=^{1 and}^mW>0.5 TeV the W widthisapproxi- mately3.6%ofitsmassandthebranchingratioofthe W→^{W Z} rangesfrom1.6%to1.2%dependingonm_W.Productioncrosssec-

E-mailaddress:[email protected].

tions in pp collisionsat √

s=^{8 TeV for}^the ^W ^bosonâs^well âs theWwidthandbranchingratiosofW→^{W Z} ^forâ^selectionôf W bosonmassesintheEGMwithscalefactorc=^{1 are}^givenⁱⁿ Table 1.

SearchesforaWbosondecayingtoνhavesetstrongbounds on the mass of the W when assuming the sequential standard model (SSM) [17,18], which differs from the EGM in that the WW Z coupling is set to zero. Forc∼^{1 the} êffect ôf^this ^cou- plingontheproductioncrosssectionoftheWbosonattheLHC isverysmall,thustheproductioncrosssectionoftheWbosonin theSSMandtheEGMis verysimilar.Moreover, duetothesmall branching ratiooftheW→^{W Z} ⁱⁿ^theÊGM^with^the ^scale^factor c∼^1, ^the ^branching ^ratios ôf ^the ^W ^boson ^to ^fermions âre approximatelythesameasintheSSM.Nevertheless,modelswith narrowvectorresonanceswithsuppressedfermioniccouplingsre- main viable extensions to theSM, andthus the EGM provides a usefulandsimplebenchmarkinsearches fornarrowvectorreso- nancesdecayingtoW Z.

The ATLAS andCMS Collaborations haveset exclusionbounds on the productionand decayof the EGM W boson. In searches usingtheν (≡^e,μ) channel,theATLAS [19] andCMS[20]

Collaborations have excluded, at the 95% conﬁdence level (CL), EGM (c=¹⁾ ^W ^bosons ^decaying ^to ^{W Z} ^for ^W ^masses ^below 1.52 TeVand1.55 TeV,respectively. InadditiontheATLAS Collab- oration has excluded EGM (c=¹⁾ ^W ^bosons ^for ^masses ^below

http://dx.doi.org/10.1016/j.physletb.2016.02.015

0370-2693/©²⁰¹⁶^CERN^for^the^benefitôf^theÂTLASCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

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1.59 TeVusing the q^q¯ ^[21] ^channel, ând^below ^{1.49 TeV} ûsing theνqq¯[22]channel.Thesehavealsobeenexcludedwithmasses between1.3and1.5 TeVandbelow1.7 TeVbytheATLAS[23]and CMS[24]Collaborations,respectively,usingthefullyhadronicfinal state.

Diboson resonances are also predicted in an extension ofthe original Randall–Sundrum (RS) [8–10] model with a warped extradimension.Inthisextension totheRSmodel[25–27],theSM fields are allowed to propagate in the bulk of the extra dimension,avoiding constraintson the original RSmodelfrom flavour- changing neutral currents and from electroweak precision mea- surements.Thisso-calledbulk-RS modelischaracterised bya di- mensionlesscouplingconstantk/M¯_Pl∼^1,^where^kîs^the^curvature of the warped extra dimension, and M¯_Pl=^MPl/√

8π is the re- ducedPlanckmass.InthismodelaKaluza–Klein excitationofthe spin-2graviton,G^∗,candecaytopairsofW orZ bosons.Forbulk RS models with k/M¯_Pl=^{1 and} ^for ^G^∗ ^masses ^between ^0.5 ^and 2.5 TeV, the branching ratio of G^∗ to W W ranges from 34% to 16% and the branching ratioto Z Z ranges from 18% to 8%. The G^∗ widthrangesfrom3.7%to6.2%dependingontheG^∗ mass.Ta- ble 1 lists widths, branching ratio to W W and Z Z for G^∗, and productioncross sectionsin pp collisions at 8 TeVin thesebulk RSmodels.

The ATLAS Collaboration has excluded, at the 95% CL, bulk G^∗ →^{Z Z} ^with ^masses ^below ^{740 GeV,} ûsing ^the qq¯ channel[21],aswell asbulk G^∗→^{W W} ^with^masses^below^{760 GeV,} usingtheνq^q¯ ^channelâssuming^k/M¯_Pl=¹^[22].^The^CMS^Collab- orationhasalsoexcluded atthe95% CLtheG^∗ oftheoriginal RS model,decayingtoW W andZ Z withmassesbelow1.2 TeVusing thefullyhadronicfinalstate[24]andhassetlimitsontheproduc- tionanddecayofgenericdibosonresonancesusingacombination ofqq,¯ νqq¯ andfullyhadronicfinalstates[28].

To improve the sensitivity to new diboson resonances, this article presents a combination of four statistically independent searchesfordibosonresonancespreviouslypublishedbytheATLAS Collaboration [19,21–23]. The searches are combined while con- sideringthe correlations betweensystematicuncertainties inthe differentchannels.Thefirstsearch,sensitivetochargedresonances decaying to W Z, uses the ν [19] final state. The second search,sensitivetochargedresonancesdecayingtoW Z andneu- tralresonancesdecayingto Z Z,usestheqq¯ finalstate[21].The thirdsearch,sensitivetochargedresonancesdecayingtoW Z and neutralresonancesdecayingtoW W,usestheνq^q¯ ^final^state^[22].

Finally,the fourth search, sensitive tocharged resonances decaying to W Z andtoneutralresonances decayingtoeither W W or Z Z,usesthefullyhadronicﬁnal state [23].Due tothelarge mo- menta ofthebosons fromthe resonancedecay, theresonancein thischannel is reconstructed withtwo large-radius jets, andthe fullyhadronicchannelishereafterreferredtoasthe J J channel.

TosearchforachargeddibosonresonancedecayingtoW Zthe ν, qq,¯ νqq,¯ and J J channelsare combined.The result of this combination is interpreted using the EGM W model with c=^{1 as}^a^benchmark.

TosearchforneutraldibosonresonancesdecayingtoW W and Z Z theq^q,¯ νq^q,¯ ând ^{J J} ^channelsâre^combined,ând^the^result isinterpretedusingthebulkG^∗,assumingk/M¯_Pl=^1,âsâ^benchmark.

The ATLAS Collaborationhasperformed additionalsearches in whichnew dibosonresonancescould manifest themselvesasex- cessesoverthebackgroundexpectation. Intheanalysispresented inRef.[29]the,νν,qq¯ andqq¯νν finalstateshavebeen explored inthecontext ofthesearch fora new,heavy Higgsbo- son.Also, in thecontext ofsearchesfor darkmattera final state ofahadronically decayingbosonandmissingtransversemomen- tum[30],andafinalstateofaleptonicallydecaying Z bosonand

missingtransversemomentumhavebeenexplored[31].Thesead- ditional searches are not included in this combination. They are not expected to contribute signiﬁcantly to the sensitivity of the combined search duetothe lower branching ratioincaseof the leptonic channels, andthe useofonly narrowjetsincaseof the qq¯ννﬁnalstate.

2. ATLASdetectoranddatasample

The ATLAS detector isdescribed in detail in Ref. [32]. It cov- ers nearly the entire solid angle¹ around the interaction point and has an approximatelycylindrical geometry. It consistsof an inner tracking detector (ID) placed within a 2 T axial magnetic ﬁeldsurroundedbyelectromagneticandhadroniccalorimetersand followedbyamuonspectrometer(MS)withamagneticﬁeldpro- videdbyasystemofsuperconductingtoroids.

Theresultspresentedinthisarticleusethedatasetcollectedin 2012byATLASfromtheLHC ppcollisionsat√

s=^{8 TeV,}^using^a single-lepton(electronormuon)trigger[33] witha p_T threshold of24 GeV,orasinglelarge-radiusjettriggerwithapTthresholdof 360 GeV. Theintegratedluminosityofthisdatasetafterrequiring data quality criteriato ensure that all detectorcomponents have beenoperational duringdata takingis 20.3 fb⁻¹.Theuncertainty on theintegratedluminosityis ±².8%.It isderived followingthe methodologydetailedinRef.[34].

3. Signalandbackgroundsamples

Theacceptanceandthereconstructedmassspectrafornarrow resonancesareestimatedwithsignal samplesgeneratedwithres- onance masses between 200 and 2500 GeV, in 100 GeV steps.

The bulk G^∗ signal events are produced by CalcHEP 3.4 [35]

withk/M¯Pl=1.0,andthe W signal samplesare generatedwith Pythia 8.170 [36], setting the coupling scale factor c =^1. ^The factorisation and renormalisation scales are set to the gener- ated resonance mass. The hadronisation and fragmentation are modelled with Pythia 8 in both cases, and the CTEQ6L1 [37]

(MSTW2008LO[38]) partondistributionfunctions(PDFs)areused for the G^∗ (W) signal. The leading-order cross sections and branching ratios for the W andbulk G^∗ signal samples for se- lectedmasspointsandassumedvaluesofthecouplingparameters areprovidedinTable 1.

The backgroundsinthedifferentdecaychannelsare modelled with simulated event samples. The W+^{jets and} ^Z +^{jets back-} groundsaregeneratedusingSherpa1.4.1[39]withCT10PDFs[40].

A separatesampleisgeneratedusingAlpgen2.14[41]toestimate systematiceffects,usingCTEQ6L1PDFsandPythia6[36]forfrag- mentationandhadronisation.

The W+^{jets and} ^Z+jets productioncrosssectionsarescaled tonext-to-next-to-leading-order(NNLO)calculations[42].Thetop quarkpair,s-channelsingle-topquarkandW tprocessesaremod- elled by the MC@NLO 4.03 generator [43,44] with CT10 PDFs, interfaced to Herwig [45] for fragmentation and hadronisation and Jimmy [46] for modelling of the underlying event. The top quark pairsampleisscaledtotheproductioncrosssectioncalcu- latedat NNLOin QCDincluding resummationofnext-to-next-to- leading logarithmic soft gluon terms withTop++2.0 [47–52]. The

1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teraction point(IP)inthecentreofthedetector andthe z-axis alongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbe- ingtheazimuthalanglearoundthebeampipe.Thepseudorapidityisdeﬁnedin termsofthepolarangleθasη= −^{ln tan}(θ/2),andthedistancein(φ,η)spaceas

R≡

( φ)²+( η)².